The invention relates to systems and materials used for solid-state thermodynamic heat pump cycles or refrigeration cycles. More particularly, the invention relates to solid-state thermodynamic heat pump cycles or refrigeration cycles based on thermoelastic effect.
Thermoelastic cooling technology based on thermoelastic effect is known in the art. It is often referred to as elastocaloric effect when the underlying science is discussed, and referred to as thermoelastic effect when the engineering and application aspects are discussed. Similar to the vapor compression cooling technology, the thermoelastic method relies on latent heat released or absorbed during the stress induced solid-to-solid phase transition. Tests of the thermoelastic refrigerant have demonstrated cooling efficiency as high as 11.8. Low cost and the high manufacturability of this technology have the potential to transform refrigeration industry and deliver significant impact to both energy efficiency and the environment. The present invention addresses the system designs, specifically, how to effectively use the working materials (refrigerants) in order to maximize system efficiency, system fatigue life, and cost effectiveness.
Compare to other alternative refrigeration technologies such as magnetocaloric, thermal-electric, thermal-acoustic, electro-caloric, only magnetocaloric and thermoelastic methods show significant impact on energy efficiency and the environment. Of the two methods, the thermoelastic cooling promises to be more cost effective because it does not involve any expensive magnetic field or critical rare earth materials.
Thermoelastic cooling effect is directly related to the reversible solid-to-solid martensitic phase transformation. In many ways, this concept is analogous to the conventional vapor compression technology because both use stress to induce phase transformations, and both utilize latent heat to achieve cooling. The difference lies in the form of the refrigerant. It is liquid/vapor for vapor compression, and solid/solid for thermoelastic cooling.
The reversible martensitic phase transformation is a diffusionless solid-to-solid transformation and involves crystallographic shearing deformation. The high-temperature phase (austenite) has higher symmetry than the low-temperature phase (martensite). The decrease of symmetry during the transformation results in the formation of multiple variants each with its own associated shape change. When the material is cooled to transform, all of the variants are equally likely to form. The randomly distributed variants leave the material with little change of its overall shape. When a stress is applied to this mixture of variants, certain variants will be energetically favored and appear in larger amounts than the others. The result is a significant change in shape as high as 10%. When the deformed martensite is warmed, the material transforms back to its austenitic configuration, which also restores the original shape of the alloy, acting as if it has a memory, thus the name of shape memory alloy (SMA).
In addition to temperature, a martensitic transformation can also be induced directly by stress. FIG. 1 depicts the process of stress-induced martensitic phase transformation in a CuAlNi alloy. At temperatures above the phase transformation, the material is in its austenite state (A), the stress-strain curve is steep, reflecting relatively high elastic constants. When the stress reaches certain magnitude, a martensite (M) starts to appear, and the material becomes soft. At this point, a small increase of the stress results in a large amount of deformation (strain). The material remains soft until most austenite is transformed; then the material starts to recover its rigidity, and the stress-strain curve becomes steep again. The large deformation with a small increase of stress is known as super-elasticity. The modern self-expanding stenting technology is based on super-elasticity.
Currently, the most widely used shape memory alloy is Nitinol (Nickel Titanium Navy Ordnance Laboratory). It is a binary alloy serendipitously discovered in 1961 (see Document No. 1), and later understood through the dedicated work of F. E. Wang. Nitinol's austenite phase has an ordered cubic (B2) crystal structure; its martensite has an ordered monoclinic (B19′) crystal structure; and it has another intermediate rhombohedral phase (B2′) often referred to as the R phase. The latent heat of each transformation is shown in FIG. 2 (see Document No. 2).
In addition to the shape memory alloys, there exist thermoelastic polymers that are capable of transforming from one solid phase to other solid phase, absorbing or releasing latent heat during the phase transformation. The transformation may be induced by temperature, stress, magnetic field, electric field, light, solution, or other forms of energy input. Example of the thermoelastic polymer include, but are not limited to, polyurethanes, polyurethanes with ionic or mesogenic components made by prepolymer method, block copolymer of polyethylene terephthalate (PET) and polyethyleneoxide (PEO), block copolymers containing polystyrene and poly(1,4-butadiene), and an ABA triblock copolymer made from poly(2-methyl-2-oxazoline) and polytetrahydrofuran. Compared to thermoelastic metals as refrigerant, thermoelastic polymer might be more cost effective, has longer service life, and requires less critical stress; however, it has a smaller thermal conductivity and a less power density.
The thermoelastic effect is a known effect that had been studied for several decades. While most of the efforts have been focused on the applications in the field of sensing and actuation, the potential of using the thermoelastic effect for cooling or refrigeration has only been explored spottily. For example, U.S. Pat. No. 6,367,281 describes the concept of thermoelastic cooling relatively adequately and attempts to disclose a refrigeration systems based on the thermoelastic effect (see Document No. 3). To the best of the present inventor's knowledge, this patent is the only instance where the concept of thermoelastic cooling was discussed. However, the embodiments disclosed in U.S. Pat. No. 6,367,281 are based on using tensile or torsional stress to induce the phase transformation. Since the working materials have a limited fatigue life under these two types of stress (<100,000 cycles when strain is >2%), any systems constructed based on these embodiments will have a limited service life and require undesirably high costs. In comparison, a system using compressive stress has much improved fatigue life. The fundamental reason for this difference is that micro-cracks existing in the materials will propagate with tensile or torsional stress, but they will heal with compressive stress. An innovative system design utilizing compressive stress while maintaining effective heat exchange is needed.
The typical stress required to induce the phase transformation under compression is greater than 200 MPa and can be as high as 900 MPa. Applying such a large stress requires a rigid frame and a powerful loading cell. Cost-effectively applying compressive stress with a small footprint is one of the challenges for commercializing the thermoelastic cooling technology.